1. Field of the Invention
The present invention relates to a strip line filter, and more particularly to a strip line filter used in a communication circuit or the like.
2. Description of Related Art
FIG. 14 shows a conventional strip line filter 101. The strip line filter 101 has an input terminal electrode 102 and an output terminal electrode 103 on mutually opposite ends (right and left ends in FIG. 14) and ground terminal electrodes 104 and 105 on the other mutually opposite ends (front, and rear ends in FIG. 14). The ground terminal electrodes 104 and 105 are to connect strip lines provided inside the filter 101 with a common ground electrode provided on the bottom surface of the filter 101. The ground terminal electrodes 104 and 105 are so large as to cover the respective ends almost entirely. The strip line filter 101 is surface-mounted on a printed board 107. More specifically, as can be seen In FIG. 14, the output terminal electrode 103 and the ground terminal electrode 104 are electrically connected with conductors 108 and 109 on the surface of the printed board 107 respectively by solder 112 and 113 and fixed on the printed board 107. Although it is not drawn, the input terminal electrode 102 and the ground terminal electrode 105 are connected with conductors on the surface of the printed board 107 by solder in the same manner.
When the strip line filter 101 is used in a high-frequency band, residual inductances occur on the ground terminal electrodes 104 and 105. The strengths of the residual inductances depend on the volume and the area of the solder 113. Since the areas of the ground terminal electrodes 104 and 105 are large, it is very difficult to conduct the soldering so as to have a specified volume of solder 113 on a predetermined area. Therefore, the strengths of the residual inductances occurring on the ground terminal electrodes 104 and 105 will not be predicted values, and accordingly the filter characteristic of the strip line filter 101 is likely to be far from a designed one. Thus, strip line filters of this type vary to a great extent in the strengths of residual inductances on the ground terminal electrodes.